1. Field of the Invention
invention relates to an apparatus and method for exposing an object to a heated vapor, and more particularly, to a system for drying an object, such as a semiconductor wafer, by vaporizing a liquid, such as isopropyl alcohol, and exposing the object to the vapor.
2. Description of the Related Art
During fabrication, an electronic component, such as a semiconductor wafer, is normally subjected to many wet chemistry cleaning operations interspersed with rinsing steps. The rinsing is typically done with deionized (DI) water that, if allowed to dry naturally, can leave streaks, stains, or spots on the component's surface. As a result, various methods have been developed in an attempt to thoroughly dry a component's surface without leaving any contaminating residue.
One known drying method, disclosed by Bettcher et al. in U.S. Pat. No. 4,841,645, is to position a cassette full of wet semiconductor wafers into a heated dense vapor cloud of a high purity solvent, such as isopropyl alcohol (IPA). The IPA condenses on the wafers and combines with the water on the wafers' surfaces. As this liquefying process continues, the DI water loses its surfaces tension and shears off the wafers' surfaces. The cassette is then withdrawn above the heated vapor cloud and positioned in a cooling zone where the wafers dry completely. Vapor condensation formed within the chamber is removed from the chamber and processed through a purification and recovery system before it is reused.
A similar process, disclosed by Schumacher et al. in U.S. Pat. No. 5,054,210, consists of a vapor dryer system with distinct vapor and cooling zones. The semiconductor product is first lowered into the vapor zone where the replacement of the DI water with the IPA occurs. The semiconductor product is then raised to the cooling zone where the drying process is completed. The patent states that cooling in the cooling zone is desirable to return the IPA vapor to a liquid state so that it may free fall to the liquid sump to be reheated.
A somewhat different vapor drying system is disclosed by Roberson, Jr. et al. in U.S. Pat. No. 4,977,688 and U.S. Pat. No. 5,115,688. While that system does not have distinct vapor and cooling zones, it requires the drying operation to take place in a vacuum process chamber. A vacuum pump and a series of valves are used to control the internal pressure in the vacuum process chamber and the amount of IPA vapor and nitrogen that are introduced into the chamber. A fixed volume of IPA vapor is introduced into the vacuum process chamber to perform the drying operation and is evacuated from the chamber and discharged after the drying process is complete.
Similarly, in U.S. Pat. No. 4,838,476, Rahn discloses an apparatus for treating a printed circuit board by subjecting it to an unsaturated vapor cloud within a subatmospheric treatment chamber. As with Roberson, Jr. et al., the apparatus includes a chamber whose pressure is controlled with a vacuum pump. When the circuit boards to be treated are introduced into the treatment chamber via a conveyor belt, the unsaturated treatment vapor that has been generated within the chamber condenses on the boards and releases its latent heat of evaporation.
A primary drawback with the above-described Bettcher et al. and Schumacher et al. devices is that they employ a relatively stagnant vapor cloud to perform the drying or treating operation. To prevent the cloud from rising, the area above the cloud is cooled by condenser coils, causing some vapor to condense on cool surfaces and some to condense and fall into the hot vapor cloud. That condensed vapor or liquid which falls into the cloud both cools the cloud and must be reheated to be revaporized. This process is counterproductive and a considerable amount of time is required for a sufficient quantity of IPA vapor to condense on the surfaces of a semiconductor wafer in order to completely displace all of the DI water and thereby effectively dry the wafer. Lengthy drying times slow the overall process time for component fabrication.
In addition, Bettcher et al. and Schumacher et al. both require a two-step drying process--first placing the semiconductor wafer in a vapor zone and then removing it to a cooling zone. This two-step approach both complicates the drying process and necessitates additional handling of the very fragile components. Moreover, while neither Roberson, Jr. et al. nor Rahn disclose two separate processing areas, they both require processing at pressures substantially below atmospheric pressure, thereby requiring vacuum pumps and complex valving systems.
Furthermore, while Bettcher et al. provides a complex system for recycling both the IPA condensate and the water/IPA mixture, Roberson, Jr. et al. provides no mechanism whatsoever for recycling the IPA condensate.